Measuring apparatus



J1m 9 T. R. HARRISON MEASURING APPARATUS Filed 'llay 29, 1937 4Sheets-Sheet 1 mmvron THOMAS R. HARRISON BYiu-agkW A TTORNEY June 10,1941. T. R. HARRISON MEASURING APPARATUS Filed May 29, 1937 4Sheets-Sheet 2 N w R 222222.222 mm H i mu m .I...... VR. w W m 4 m T 1 9A b .6 h m 9 r 9 c 7 u\| w 9 w June 10,- 1941. 'r. R. HARRISON MEASURINGAPPARATUS Filed May 29, 1937 4 Sheets-Sheet 4 A E Q.

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moms R. HARRISON BYMMMI A TTORNEY Patented June 10, 1941 2,245,034MEASURING APPARATUS Thomas R. Harrison, Wyncote, Pa., assignor to TheBrown Instrument Company, Philadelphia, Pa., a corporation ofPennsylvania Application May 29, 1937, Serial No. 145,637

15 Claims.

The present invention relates to apparatus for ascertaining andcontinuously recording and/or controlling the magnitude of a variablecondition and more particularly the invention relates to self balancingmeasuring apparatus which is of special utility in recording and/orcontrolling temperature variations in incandescent bodies.

An object of the invention is'to provide radiantenergy responsiveapparatus for ascertaining and continuously recording temperaturevariations in incandescent bodies.

Another object of the invention is to provide apparatus of the typereferred to above, which is characterized by being extremely fast inresponse and self balancing.

A special object of the invention is to provide high speed temperaturemeasuring and recording apparatus in which a light sensitive device maybe employed as the condition responsive element,

and in which delicate electrical current measuring instruments, such,for example, as microammeters or milh-ammeters are not required noremployed.

In measuring and recording temperature variations in incandescentbodies, it is relatively important in certain applications that theapparatus employed be capable of responding to and recording thevariations in a minimum of time. Forexample, ii an accurate record isdesired of the variations in temperature throughout their length ofsuccessive pieces of material such as bars or billets passing in processthrough a steel mill, it is of especial importance that the measuringapparatus be capable of a high speed of response. The practicability ofemploying light sensitive devices or photoelectric cells in making suchmeasurements is readily apparent, since, as is well known, photoelectriccells are characterized by their extreme rapidity of response tovariations in light intensity and pass an electric current which bears adefinite relation to the temperature of the body from which the lightemanates.

Various high speed radiant energy responsive systems utilizing a lightsensitive device as the condition-responsive element have heretoforebeen proposed for measuring and recording temperature variations inincandescent bodies, but all of the proposed systems have been of adeflectional type employing a delicate electrical current measuringinstrument and a scale or chart with which the deflecting member orpointer of the instrument is adapted to cooperate. In the proposedsystems the radiant light from an incandescent body is ordinarilydirected on a photoelectric cell and the electric current passed by thecell is measured by a suitably callbrated electrical current measuringinstrument. In some instances, the current passed by the photo-electriccell is first amplified by some form of electronic amplifier and theamplified quantity is measured by a similarly calibrated electricalcurrent measuring instrument. As is apparent the position of thedeflecting member or pointer of the instrument relative to the scalethereof will provide a measure of the temperature of the incandescentbody, and, if a record is desired, suitable apparatus may be arranged incooperative relation with the instrument pointer to periodically recordits position on a chart, or a pen may be mounted directly on theinstrument pointer and arranged to cooperate with a chart to therebyprovide a continuous record of the temperature variations. Since theonly time lag in recording by the last mentioned method is that requiredfor the instrument pointer to swing up scale, this method has proved tobe the most desirable of the prior art devices adapted for utilizing toadvantage the high speed of response of the light sensitive elements.

As is well known however, deflectional measuring instruments areundesirable in many respects for making precision measurements. For example, the calibration of a deflectional measiu c instrument isdependent upon the constancy o: magnets, springs, jewel bearings, andthe level of the instrument, all of which are aifected and changed tovarying extents by many factors such, for example, as temperature, age,and vibration. Such instruments, furthermore, are limited inrespect tothe available torque for recording or controlling purposes makingnecessary the use of narrow and therefore undesirable charts andunsatisfactory control instrumentalities. Such instruments, furthermore,embody charts having non-rectangular coordinates which are undesirable,and ordinarily, the periodicity of response necessarily employed resultsin sluggish response thereof to smal-ldepartures.

The above objections have been avoided in the apparatus of my inventionby providing a potentiometric network especially adapted to be used withthe radiant energy responsive apparatus for recording the measuredvariations in a minimum of time. In the preferred embodiment of myinvention, a photoelectric cell, responsive to the radiantlightemanating from an incandescent body whose temperature is to bemeasurer, is employed to control the potentiometric balance and areversible electrical motor is arranged to network. As is apparent, thenew position of the V rehalancing contact will provide a measure of thetemperature of the incandescent body and, if desired, a pen may bemounted on the carrier of the rebalancing contact and arranged tocooper-- ate with a suitably calibrated. record chart for recording thetemperature variations.

In potentiometric measuring apparatus wherein a known or standardvoltage is compared to an unknown voltage it is ordinarily desirable tocheck the standard voltage at frequent intervals to insure the constancyof calibration. Such check ng operations, however, are not permissibleirable in high speed measuring and record ing apparatus because of theadditional apparatus and the time required for such operations.Moreover, the standard voltage is ordinarily supplied from suitablebatteries provided especially for purpose. In this respect, my inventionhas additional practical utility in that the poten 'tiometer voltage maybe supplied from any available ccrnmercial source of electric currentsince means have been provided for automatically compensating forfluctuations in the supply line voltage.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of specification. For a better understanding of the inventionhowever, and the advantages possessed by it, reference should be made tothe accompanying drawings and descriptive matter in which I haveillustrated and described preierred embodiment of the invention.

f the drawingsi Fig. 1 is a diagrammatic view illustrating oneembodiment of the invention as adapted for use in measuring andrecording the temperature of an incandescent body;

Fig. 2 illustrates a modification of a portion of the device of Fig. l;v

Figs. 3-6 illustrate further modifications of the device oi Fig, l; and

Fig. '7 is a diagrammatic representation of the use of the invention ina control system.

In Fig. i of the drawings, a photoelectric pyrometer is shown in which aphotoelectric cell i is arranged to receive light from the interior of afurnace or from a incandescent body 2, and a second photoelectric cell 3is arranged to receive light from a lamp i, the illumination of whichadapted to controlled by means responsive to the relative conductivitiesof the two photoelectric cells. The photoelectric cells are ar ranged inan electrical bridge circuit and as the temperature of the incandescentbody varies, the illumination of lamp 5 is varied until the illuminationon the two photoelec'ric cells is balanced. The magnitude or the lampenergizing current will then be a measure of the magnitude of thecondition. I

As shcvm the two photoelectric units are connected in series relation bya conductor 5 and the series arrangement is'connected across a suitableportion of a voltage divider 6 by conductors 7A and 8A. The unit ispreferably enclosed in a suitable casing (not shown) and the casing isarranged in such manner that the only light which impinges on the unitis that which is to be measured. Desirably the unit 3 is also suitablyenclosed and arranged so that it is subject only to light from the lampi.

The voltage divider E is energized from a transformer l whichcombination step-up and step-down transformer and comprises a linevoltage primary winding ll connected to the supply lines L and L lowvoltage secondary windings 9, ill, 6!, and i2, and high voltagesecondary windings l3 and M. The low voltage secondary winding 8 isconnected by conductors i5 and Hi to the heater filament ill of anelectronic valve l1 and supplies energizing current thereto. Theelectronic valve ll is a heater type triode and includes an anode i8, agrid it, a cathode 20, and the heater filament 2i. As shown, the anodeE8 of valve ii is connected to the positive terminal of the divider l.by a conductor through a resistor .23, the cathode 2E3 is connected to apoint on the voltage divider midway between the points of connection ofthe conductors EA and 8A thereto, grid 09 is connected by a conductor El to the-conductor 5 between the two photoelectric cells.

The low voltage secondary winding i2 is connected by conductors and 28to the heater filament or cathode of 2V second electronic valve andsupplies energizing current thereto. The electronic valve i a filamenttype triode and includes an anode Ell, a grid 29, and the filamentcathode Anode is connected by a. conductor St to the positive terminalof a second voltage divider which is energized from the transformer T1in a manner to be presently explained and the negative terminal thereofis connected to the anode of valve W. A center point on the :fdamentcathode 539 is connected to a point on the voltage divider 32 which isnegative with respect to the noint of connection of the anode 28 theretoand has connected in circuit therewith the filament of lamp l and aresistor 38. The circuit from cathode St to the voltage divider hetraced from cathode 30, conductor lamp 3, conductor Bl, resistor 38, andconductor to the voltage divider 32. The lamp 3 is thus arranged to beenergized by the anode current flow through the valve ill. As shown, thegrid 29 of valve ill is connected to the lower end oi the resistor asseen in Fig. 1 so that the flow oi anode current through the valvevaries the potential of grid 29 and thereby the anode current flowthrough lamp G will vary accc ingly.

Electronic valves 0-6 and it are also provided, which valves may heconventional full wave rectifiers and are employed to maintain directcurrent potentials across the voltage dividers 6 and 32, respectively.The filament cathode 43 of valve is connected by conductors id and 65 tothe low voltage secondary winding id and receives energizing currenttherefrom. The anode plates 4| and $2 of valve 3d are connected byconductors and with the terminals of the high voltage transformersecondary winding lit, and the rectified current is connected across theterminals of a filter M. A center tap on the secondary winding it isconnected by a conductor to one terminal or the filter 53 and the otherterminal of the latter is connected to the filament cathode 43 so that adirect current voltage is maintained across the filter. The terminals ofthe filter 54 are connected to opposite ends of the voltage divider 6 sothat a steady direct current voltage is maintained across the dividerwhich, due to the filtering action of the filter, is substantially freefrom alternating components.

Similarly, the filament cathode 49 of valve 46 is energized from the lowvoltage secondary winding ll through conductors 50 and the anode plates41 and 48 are connected by conductors 56 and 51 with the terminals ofthe high voltage secondary winding l4, and the rectified current isconnected across the terminals of a filter 58. A center tap on thesecondary winding I4 is connected by a conductor 59 to one terminal ofthe filter 58 and the other terminal of the latter is connected to thefilament cathode 49 so that a direct current voltage is maintainedacross the filter. The terminals of the filter 58 are connected toopposite ends of the voltage divider 32 and due to the filtering actionof the filter a steady direct current voltage substantially free fromalternating components is maintain across the divider 32.

When light from the incandescent body 2 is focussed on the photoelectriccell I, this unit accordingly becomes more conductive and therebyrenders the grid IQ of valve II more positive relative to the cathode 20resulting in'an increase in the current passing through it and aconsequent increase in the potential drop in resistor 23. Thereupon thegrid 29 of valve 21 becomes more positive relative to the cathode 30,resulting in an increase in the current conducted by the valve 21 andthereby through the lamp 4. As lamp 4 subsequently brightens, thephotoelectric cell 3 becomes more conductive and drives the grid I! ofvalve ll less positive, and since valve 61 controls the potential ofgrid 29, the latter is also made less positive. This establishes abalanced condition of the illumination of the two photocells, and theenergy used by the lamp 4 is then an indication of the value of thelight being received by the unit I.

When photoelectric cells of the vacuum type are employed, this balanceof illumination will be exact to a relatively high degree of precisionby virtue of the fact that when the voltage across the photoelectriccells is high enough to draw all of the electrons from the cathode tothe anode the currents conducted by the photoelectric cells will beconstant for all higher voltages. Accordingly only slight unbalance ofillumination will be sufiicient to swing grid l9 to any degree of biasthat may be necessary to restore the balance oi illumination.

As was previously noted the current which passes through the lamp 4 alsopasses through the resistor 38 so that a potential drop is maintained.across the resistor 38 which varies in accordance with the illuminationreceived by the photoelectric cell l and thereby in accordance with thetemperature of the incandescent body. In the preferred embodimentof myinvention, this potential drop is opposed to and normally balanced by ameasured part of the potential drop maintained across a slidewireresistor 6| by a voltage compensating bridge circuit 60. The

importance of maintaining the potential drop across the resistor 6|constant and independent of changes in line voltage will be readilyappreciated when it is understood that the voltage drop across resistorBI is employed as a standard with which the voltage drop across resistor38 is compared. Any change 'in voltage across the slidewire resistor BIis, therefore, not permissible since such change would obviously affectthe calibration and stability of operation of the instrument.

As shown, the voltage compensating bridge 60 is energized from asuitable direct current source through conductors 62 and 53 connected toone pair of conjugate points, and includes resistors 64, 65, and 66 asthree of its arms, and a glow discharge tube 61 as the remaining arm.The slidewire resistor 6| is connected to the other set of conjugatepoints. The glow discharge tube 61 comprises two spaced electrodes in asuitable envelope containing inert gas at a pressure such that the tubehas a substantially constant drop thereacross when in a continuouslyconducting state. The resistor 64 is desirably so proportioned that thevoltage drop across the glow discharge tube is adjusted to the properoperating value, at which adjustment the voltage across the glowdischarge tube will vary only slightly as the voltage of the sourcechanges. By properly proportioning the resistors 64, 65, and .6, forexample, by making the resistor 65 large in comparison to resistors 64and $6, I have found the potential drop across the resistor 66 may bemade to vary by the same amount that the potential drop across the glowdischarge tube does for the same change in the voltage of the source.Thus, as the line voltage changes, the potential of both ends of theslidewire resistor 6| will rise and fall simultaneously in unison withthe line voltage change with the result the voltage drop across theslidewire resistor will remain constant. This arrangement for producinga constant source of direct current potential is being claimed in mycopending divisional application Serial Number 193,259 filed March 1,1938.

The source from which the voltage compensating bridge is energized maybe any direct current source of voltage and as shown in Fig. 1 it may beenergized through a suitable rectifier circuit from a transformer 68which is a combination step-up and step-down transformer and comprises aline voltage primary winding '0 connected to the supply lines L andLF,low voltage secondary windings l0 and II, and high voltage secondarywindings I2, 13 and 13a- The rectifier circuit referred to may besimilar to the rectifier circuits including the rectifier valves 40 and46, and includes a rectifier valve 14 having a filament cathode 15connected by conductors l8 and 19 to the terminals of the low voltagetransformer secondary winding 10 and receiving energizing currenttherefrom. The anode plates I6 and 11 of valve I4 are connected byconductors and 8| to the terminals of the transformer secondary winding12. A center tap on the secondary winding I2 constitutes the negativeterminal and the filament .15 constitutes the positive terminal of therectifier. The positive terminal of the rectifier is connected to oneenergizing terminal of the voltage compensating bridge 60 through asuitable resistor 82 by the conductor 62 andthe negative terminalthereof is connected by the conductor 63 to the other. energizingterminal of the bridge. If desired. a suitable filter may be connectedbetween the rectifier and the voltage compensating bridge 60, but I-havefound that it is not necessary to supply pure direct current voltage tothe bridge 60 in this arrangement and that very satisfactory operationis had when full wave rectified pulsating current is supplied to thebridge.

It will thus be noted the potential drop across the resistor 38 isarranged to be opposed to and balanced by a variable portion of thepotential drop across the slidewire resistor 6i which remainssubstantially constant even with wide variatlons in the potential of thesupply lines L and U. As shown, the resistor 38 is connected in a seriescircuit with the slidewire resistor 6! Which may be traced from thenegative side of resistor 38, conductor B ll, to the negative end ofslidewlre resistor ti, slidewire contact 85, conductor 56, resistors 8?and til, conductor a pair of induction generator windings till and ti,and conductor to the positive side of resistor 38. The point ofconnection of the resistors 87 and Q is connected through a cathodebiasing resister to the cathodes t8 and lilll of a pair of electr valvesQ l and 99, and opposite ends of the are connected to the valve rids illGUS, respectively.

The valves 'l type tr in the s vert the cl the resistors Si and. intoalternating currents which may be further amplified and employed toselectively control the energization of a reversible alternating currentmotor. The valve il includes the cathode which be heater an anode a gridEll, and a heater filament Q8, and the valve 9d includes the cathodelFiJ, an anode a grid tail, and a heater filament ls'shown, the heaterfilaments 98 and 583 are connected in parallel relatlon and receiveenergizing current through conductors from the low voltage transformersecondary Winding ii.

The anodes 95 and are connected to pposite terr 'nals of the primarywinding Hill of a transformer lfili by conductors tilt and 6635, respec"'vely. A center tap on the transformer priwinding i connected by aconductor let? to one terminal of the transformer secondary winding andthe other terminal of the secondary wind 1 is connected by a conductorits to the negative end of the resistor 93 and therethrough the cathodesE and Hill. Thus altervoitage is supplied to the anode circuits oi thevalves 93 and as and the pulsating current conducted by these valves isadapted to be varied by the flow of direct current through the resistorsG? and The result is that pulsating anode current will flow through thetransformer winding l which will vary in phase and magnitude inaccordance with the direction and magnitude of the direct current flowthrough the resistors Bi and iii and thereby an alternating voltag willbe induced in the secondary winding lid of transformer 608 which willalso vary in phase and amplitude accordingly.

W n the grids ill and. hi2 are at thesame through opposite halves of thetransformer primary winding it" will induce magnetomotive forces in theormer which are equal and opposite with the result that no voltage willbe induced in the secondary Winding lid. When the pater ial across theresistor 36 becomes less than that across the variable portion of theslide resistor hi connected in circuit, an unwill flow through resistors87 in downward direction as seen in l and the potential of grid E willthen become less negative with respect to the cathode W3 and the grid 9will become more negative with respect to the cathode The triocle 99 istheredes as shown, are preferably contained e envelope their purpose isto conbalance d ect current flows through by rendered more conductiveand the triode 94 is rendered less conductive resulting in the inductionof an alternating voltage of corresponding phase across the terminals ofthe transformer secondary winding Ilil. As will be apparent to thoseskilled in'the art either resistor 81 or 88 may be omitted and theremaining resistor may be made zero in value without fundamentallychanging this principle of operation.

The alternatin voltage so induced in'the sec-- ondary winding l N5 isimpressed on the input circuits of a second pair of electronic valves li t and iii. The valve ill includes a cathode iii, an anode Hit, ascreen grid lit, a control grid 1': l5, and a heater filament M6, andthe valve ill includes a cathode H8, an anode H9, ascreen Mt, a controlgrid til and a heater filament As shown the heater filaments H6 and E22are connected in parallel relation and receive cue-r gizing current fromthe low voltage transformer secondary winding ll through conductors andEMA. One terminal of the transformer ondary windin till is connected tothe co. grid lit, the other terminal is connected to the contral gridH5, and a cathode biasing resistor N28 is connected between a center tapon the secondary winding lit; and the cathodes and till which, as shown,are connected together. The anodes it" M9 are connected to oppoo andsite terminals of the primary winding Hid oi a transformer E23 byconductors 926 and 92?, spectively. A center tap on the transformerprimary winding 128, is connected by a conductor H29 to the positiveterminal of the rectifier and the negative terminal of the rectifier isconnected to the negative end of the cathode biasing resistor E28 byconductor 32 and therethi'ough to the cathodes ll? and lit so that fullwave pulsating rectified current voltage is maintained on the anodecircuits of the valves Hi and Hi.

In operation when an alternating voltage appears across the terminals ofthe transformer secondary winding lit, the potentials of grids M5 andtill are swung, in opposite phase, at a frequency corresponding to thesupply line ire-- quency and thevalves i ii and i ll are eachalternately rendered conductive and non-conductive, one valve beingconductive while the other non-conductive. The resulting pulsatingcurrent flows through the transformer primary winding lid insuccessively opposite directions through the opposite halves of thetransformer result in the induction of an alternating voltage of linefrequency in the transformer secondary winding lit; whose phase andamplitude is determined by the direction and magnitude of the unbalancecurrent flows through the resistors ill and JG.

The terminals of the transformer second winding i25 are connected to onephase wind E38 of a two phase rotating field motor 6 conductors M2 andand the other phase ing i3 3 of the motor is connected to the st lines Land L through a suitable condenser Due to the action of cond r 535 thecur flow through the motor win 6336 will leac line current by appr ately96 and since motor winding lii is arr enged to be energi currents inphase or out of phase wit line current, the magnetic is set up by twindings will be displaced approx in space with the result that a rota"field will he set up in the motor in one dire or the other and the motorrotor: will accoi'di be rotated in a correspon g direction. dependingupon the direction of the po mnetric unbalance current flow through theresistors I1 and II, the motor will be selectively energized forrotation in one direction or the other, and it will be apparent themotor speed will be directly dependent upon the magnitude of theunbalance current flow.

As shown, condensers I46 and I41. are conthe transformer I23 and itsload consisting of the motor windings so that the current flows throughthe motor winding III will be at a maximum value for any unbalancecurrent flow through the resistors 8'! and 8!. i

As shown in Fig, l the shaft of motor I30 is connected in any convenientmanner to a shaft I36, which may desirably be a rod having a spiralgroove thereon, and the potentiometer contact is mounted on a carriagecarried by shaft I36 and is adapted to be moved in one direction or theother as the shaft I36 is rotated. Thus, when the motor I30 is energizedfor rotation as a result of a change in the potential drop along theresistor 38, the motor effects an adjusting movement of the contact 85along the slidewire resistor 6| in the proper direction until theeffective potential drop along the slidewire resistor is equal to thatacross resistor 38. The

unbalance current flow through the resistors 81 and 88 will then bereduced to zero and the motor will come to rest with the contact "at anew position along the slidewire which position will then be a measureof the value of light falling on the photoelectric cell I and thereby ofthe temperature of the incandescent body 2.

Desirably, a penv mounted on the carriage which carries thepotentiometer contact 85, is arranged to cooperate with a recorder chartI31 and to thereby provide a continuous record of the temperature of theincandescent body 2. The recorder chart is adapted to be driven & acontinuously rotating roller I38 and the latter is driven in anyconvenient manner, as for exampie, by a unidirectional motor I" throughsuitable gearing I4IIA, so that a record of the temperature of theincanducent body will be had as a continuous line on the chart.

In order that the speed of motor III! may be as great as possible duringrebalancing without overshooting of the balance point and consequenthunting taking place, means have been provided to ensure that the motorspeed and rate of change of its speed is substantially proportional tothe extent of unbalance. This resuit is obtained in Fig. l byintroducing into the network, in series with the unbalance voltage, avoltage whose magnitude is a function of the motor speed. thoughobtained electrically herein is substantially the same as that disclosedin my prior Patent 1,827,520, issued Oct. 13, 1931, in which Thisdamping feature althe rate of rebalance of a potentlometric measuringcircuit and arresting of the motive means when the rebalancing movementhas been proportional to the unbalance are controlled by mechanicalmeans. Thus, as the slidewire contact II approaches its new balanceposition, the unbalance voltage will decrease in value and if the motorspeed is then such that it would ordinarily coast beyond the balanceposition due to its Inertia, the opposing voltage which is introduced incircuit will be appreciably greater than the unbalance voltage and willtend to effect energization of the motor in the opposite direction andthereby produce a positive damping action which will check the motorspeed before balance is reached and gradually reduce it to zero as theunbalance is reduced to zero. Specifically a pair of opposed windingsand SI of an induction disc generator I40 are connected in seriesrelation with the resistors 38, SI, 81 and 88, and the windings 90 andiii are so arranged that the generated voltage will be opposed to theunbalance voltage between the potential drops across the resistor 38 andslidewire BI.

The induction disc generator I40 comprises a rotatable aluminum orcopper disc I which may be driven directly from the motor shaft orthrough suitable gearing, as desired, having a winding I42 connected byconductors I43 and I44 to the terminals of the' transformer secondarywinding 13a and receiving energizing current therefrom. The winding I42is arranged on one side of the disc in such manner that the alternatingmagnetic flux set up by it will pass through the disc, and the opposedwindings 90, and BI are arranged side by side on the opposite sideof'the disc in such positions relative to the winding I42 that when thedisc I is stationary equal numbers of lines of the alternating magneticflux set up by winding I42 will pass through them. Thus equal andopposite alternating voltages will be induced in the windings 90 and BIwith the result that normally the resultant damping voltage will bezero. When the disc I is rotated, however, the alternating magnetic fluxset up by the winding I42 is distorted, and depending on the directionof rotation of the disc, more lines of flux will pass through oneopposed winding 90 or 9| than the other, resulting in the appearance ofan alternating voltage across the terminals of these windings.

This induced voltage will be substantially in phase with the linevoltage or displaced therefrom depending upon the direction of rotationof the disc I, and as is apparent, the amplitude of the induced voltagewill be directly dependent upon the speed of rotation of the disc. Forexample, if the speed of rotation of the disc is small, the degree ofdistortion of the alternating magnetic field set up by the winding I42is correspondingly small, andas a result the voltages induced in thewindings 90 and BI will be nearly equal so thattheir resultant voltageis small. As the speed of rotation of the disc be,- comes greater,however, the degree of distortion of the magnetic field will becomegreater, and

thereby the voltage induced in one or the other tentials due to theaction of the cathode biasing resistor a! and thereby reduce thepulsating current flows conducted by the.valwes to nelllfl values. As Isreadily apparentthe motor III will then be dee er-shed. but if ithappenedtobe speeding when the contact 85 and slidewire resistor 6ibecame disengaged. it may coast beyond the new balance position sincesuch disengagement also results in removal of the damping voltageintroduced into the network by the induction disc generator Mil. Ifcontact between the slidewire resistor 6i and contact 85 is sub-'sequently made, as is normally the case, the motor will be energized forrotation in the opposite direction and effect adjustment of the contact85 back to the balance position, but it should be noted the recordingpen will have followed and recorded the hunt oi the motor so that a truerecord of the temperature variations of the incandescent body will notbe had.

An obvious method of overcoming this objection is to make the contactpressure between the slidewire resistor i and contact great enough toprevent disengagement therebetween. While effective, this method,however, is not the most desirable one in certain applications becauseof resulting frictional wear between the contact and resistor. Anotherand more desirable method is that illustrated, more or lessdiagrammatically, in Fig. 2. For purposes of simplification only, aportion of the device of Fig. i has been shown in the modification ofFig. 2, In this modification it will be noted the induction discgenerator windings 9B and di are connected in a local cir cuit includingthe grids and Hi2, but not the resistors 33 and (ii. As seen in Sig. 2one terminal or" the arrangement comprising the generator windings 9dand Si connected series is connected to the grid 9? and the otherterminal is connected to the bottom end of resistor 68. It will beunderstood the windings Eli and Bi may just as readily be connected attween the grid H32 and the top end of esistor 8T In this arrangement itwill be seen that if, during rebalancing, the contact should bedirection by the amplified quantity of the damp ing voltage and willquickly come to rest. the contact 35 subsequently engages the slidewireresistor iii and the balance position is not yet reached, the motor willbe energized for rota tion in the direction in which it was previouslyrunning. and due to the action of the damping voltage will come to restprecisely on the balance position. Thus much smaller contact pressuresmay be employed between the contact and resistor iii in the arrangementof Fig. 2 than in the arrangement of Fig, l and consequently the life ofthe contact and resistor may be materially increased.

In commercial applications it is usually desirable to provide means forfacilitating the necessary calibrating and adjusting operations, and asshown means have been provided especially for this purpose in themodification illustrated in Fig. 2. As shown a resistor M5 is connectedin shunt to the resistor 38 for adjusting the potential drop across thelatter to a desirable value and suitable means have been provided in thebridge circuit 83 for adjusting the potential drop across. the slidewireresistor hi to a correspondingly desirable value. The latter meansincludes a resistor ia connected in series with the resistor 6i and acooperating contact i-tiib, Contact 1452) is connected by a conductor M8to a contact I49 which is arranged to be moved alon the bridge resistor66 for precisely adjusting the voltage compensation of the bridge 80. Ihave found that by varying the position of, contact M9 along resistor 66full compensation for line voltage variations, or under compensation orover compensation may be had as desired. Means have also been providedin the form of a condenser I50 connected in shunt to resistors iii and1450: for filtering out any alternating components in the current flowthrough resistor Furthermore, provision has been made in thisarrangement in the form of a resistor |5|A connected in shunt to theinduction disc generator windings and iii and a cooperating contact i52Afor adjusting the damping voltage introduced into the network to adesirable value.

In Fig. 3, I have illustrated a further modification of the device ofFig. 1 wherein an additional stage of amplification energized from asuitable direct current source which may desirably be the potential dropacross voltage divider B2, is provided for measuring the potential dropacross resistor 38. As shown, the additional stage of amplificationreferred to includes a pair of ele tronic valves i5i and I56. The valvesand l56, which may desirably be included in the same envelope, areheater type triodes. Valve cludes a cathode i'52, an anode control gridi5 1, and a heater filament valve i56 includes a cathode itl, an anodeit a grid L59, and a heater filament ll l As shown the valves iEii andare connecte in parallel relation with the cathodes lllii .1

idl' connected together, and the positive ends resistors lti and i652,which. are connected ii. the anode circuits of valves and tilt, resectively, are connected toge' r The pot ".l drop across resistor 38 isimpressed on the grid circuit of valve iiii and the potential dropacross a variable portion of the slidewire resistor GE is impressed onthe grid circuit of valve Hit. The heater filaments may be connected inparallel with the heater fil and 9% of valves Sit and and reachtrier-gin ing current from the transformer secondary winding ll or aseparate transformer sec Winding may be provided for this desired. AnodeVoltage is supplied to from the terminals or" voltage divider w 102 maybe energized from. a filter as shown in Fig, l, and as shown, the anodesit??? are connected together through resistors ill and 88 by conductorsand. anodes are connected to the positive side of voltage divider 32 andthe cathodes are connected through cathode biasing resistor list to thenegative si It will be noted that by connecting the resistor. Hill to apoint on the voltage divider which is more negative than the point ofconnection of the resistor 38 thereto that the change in bias on gridsltd and 559 required to effect balance of the system for the maximumchange in volt age across resistor 38 is much less than it would have tobe if both of these resistors were connected to the same point on thevoltage divider.

When the grids I54 and 559 are at the same potential. the Valves HI and555 will be equally conductive or substantially so and the potentialdrops across resistors NH and 162 connected in the anode circuits of thevalves will be equal so that the anodes I53 and I53 will be at the samepotential, and thereby the grids 9i and H12 of valves 94 and 99 will beat the same potential. Upon variation in the light falling upon thephotoelectric cell I, the current conducted by valve 21 will be variedas hereinbefore explained, and a change in potential across the resistor38 will result. As is apparent, the potential of grid I54 will then alsobe varied with respect to the cathode I52 with the result that theconductivity'of valve II will be rendered diiferent from that of valveI56 and thereby the potential of anode I55 will be rendered difierentfrom that of anode I55. Thus a slight inequality between the potentialdrops across resistor 35 and the efiective portion of resistor 5!produces a magnified potential difference between the anodes I55 andI55. Since the grids 9" and I0! are connected to the anodes I55 and I55,respectively, one of the valves 54 or 99 will be rendered moreconductive than the other, and thereby will cause the motor I30 to beenergized for rotation in one direction or the other as explained inconnection with Fig. 1. The contact 55 may desirably be connected to anddriven by the motor shaft in this arrangement as in the arrangement ofFig. 1 so that the resulting energization of motor I50 may be employedto effect an adjustment of the contact 85 along resistor 6| in theproper direction to change the potential on grid I59 to correspond tothat on grid I 54 and thereby reduce the unbalance current flow throughthe resistors 81 and 50 to zero.

In this arrangement as well as in the arrangements of Figs; 1 and 2,means have been provided for arresting the motion of the motor when themovement of contact 55 has been proportional to the change in the valueof light falling onthe photoelectric cell I. Although an induction discgenerator I40 of the type shown in Figs. 1 and 2 may conveniently beemployed for arresting the motion of motor I30, it'will be noted that inthis modification a four pole generator I40 v is provided for thispurpose, A winding I42 which may be energized from a suitable source ofalternating current, for example, from the transformer secondary winding15a of Fig. l, is wound on two diametrically opposite poles and awinding 50 is wound on the remaining pair of diametrically oppositepoles. A rotor I which is made up of iron laminations forced into acopper cylinder, is arranged to be rotated between the poles andoperates as does the disc I of the induction disc generator I to distortthe magnetic field set up by the winding I42 When the rotor I is at restit will be seen that substantially no lines of alternating magnetic fluxthread the winding 55 so that the voltage across the terminals of thelatter winding is zero. When the rotor I is rotated, however, as by themotor I50, for example, the alternating magnetic flux set up by thewinding I42 will be distorted and as a result lines of magnetic fluxwill thread the winding 55 and induce a voltage in the latter of phaseand magnitude corresponding'to the direction and speed oi'rotation ofthe rotor I4I As shown a resistor I5I is connected across the terminalsof the winding 55 and a variable portion of the resistor is connectedbetween the grid I54 of valve I5I and the positive end of resistor 55.

Thus, as the motor I55 gains speed, a counter voltage will be introducedinto the network which tends-Io drive the grid I54 in the oppositedirection from that in which it was driven by thechsngeinthevalueoflisht faliingontnephotoelectric cell I with the resultthat as the balance pofltion, h reached, the motor speed is quicklybecomes acre in value when the grids I 54 and I55 are again at the samep tential. It should be noted that a fundamental distinction whichexists between the induction disc generators I40 and I4II is that whenthe induction disc MI is at rest zero voltage is induced across theterminals of windings and SI because the voltage induced in winding bythe alternating magnetic fiux threading it, is

equal and opposite to the voltage induced in winding III by thealternating magnetic flux threading the latter, whereas no voltage isinduced in winding 90 of generator I45 when rotor MI is at rest byvirtue of the fact that no lines of alternating magnetic flux thenthread winding 90 The generator H0 is thus seen to operate in a mannersimilar to the modification of the induction disc generator I40considered hereinbefore.

In Fig. 4, I have illustrated, more or less diagrammatically, a furthermodification of my invention in which only one photoelectric cell isemployed in measuring the. temperature of the incandescent body 2. Itwill -be noted that in this arrangement, a pair of electronic valves INand I56 are connected in the manner described in connection with Fig. 3for varying the .potentials on grids 91 and I02 of valves 54 and 55,.respectively, upon variation in the value of illumination received by aphotoelectric cell I15 and thereby inducing an alternating voltage inthe transformer secondary winding I III which may be further amplifiedand applied to one phase winding of a motor I30 for causing selectiverotation thereof. In this arrangement, the potentials on the valve gridsI54 and I55 are not controlled in the same manner, however, as they arein the Fig. 3 arrangement for recording the variation in the temperatureof the incandescent body I.

As shown in Fig. 4 the grid I54 of valve I5I is connected by a conductorI54, in which a resistor I55 is inserted, to the negative end of aslidewire resistor iii. The slidewire resistor H is desirably connectedin a voltage compensating bridge 50 which may be similar to thecorrespondingly identified part of Fig. 1. The voltage compensatingbridge 50 of Fig. 4 may be energized from any suitable direct currentsource and is also connected by a conductor I55 to the anode of thephotoelectric cell "5, the cathode of which is connected to the negativeside of filter I55. The cathodes I55 and I51 are connected together andthrough a resistor I55 to the negative side of filter I55.

It will thus be apparent the relative potentials on grids I54 and I55will be substantially identical even with line voltage variations whenthe system is balanced. In operation, the potential on grid I54 isdetermined by the potential of the negative end of the slidewireresistor 5|, and the potential on grid I55 will be determined by thepotential of the negative end of the slidewire resistor 5| plus or minusthe difference between the potential drop across the elective portion ofthe slidewire resistor 5i and the potential drop across resistor I68produced by the flow of current conducted by photoelectric cell I10.

When light from the incandescent body 2 is iocussed on the photoelectriccell I'Ifl, this unit accordingly conducts more current and therebyincreases the potential drop across resistor I68 and renders the gridI59 of valve I56 more negative than the grid I54 of valve II, resultingin a decrease in the current conducted by valve I55 and an increase incurrent conducted by valve 255 by virtue of the change in the potentialdrop across the cathode biasing resistor I63. The potential on the anodeI58 of valve I56 will then be more positive than the potential on theanode l53 of valve I5I and as a result the grid 91 of valve 94 will berendered less negative with respect to its corresponding cathode 95 andthe grid it? of valve 99 will be rendered more negative with respect toits corresponding cathode Mi resulting in the appearance of analternating voltage across the terminals of the transformer secondarywinding Mil as described in connection with Fig. 1. This alternatingvoltage may then be further amplified and the amplified quantity appliedto the phase Winding I3I of motor I as shown. The motor phase windingsare so arranged in relation to the remainder of the circuit that theresulting rotation of the motor will then be in the proper direction toadjust the contact 85 in an upward direction as seen in Fig. 4 tothereby raise the potential on grid I59 and consequently to reduce theunbalance current flow through the resistors and 8B. The motorenergizing current will thus be reduced to zero and subsequently themotor will come to rest.

The new position of the contact along the resistor 6i will then be ameasure of the value of the illumination falling on the photoelectriccell lid and thereby of the temperature of the incandescent body 2 andif desired, a chart till as shown in Fig. i may be arranged in relationto the slidewire resistor 6i and a pen may be mounted on the carrier or"the contact 35 for recording the variations in the temperature of theincandescent body. A fundamental characteristic of a vacuum typephotoelectr c cell is that the current conducted by the cell is afunction of the light impinging on it and is independent of the voltageapplied across the cell provided the voltage is sufficient to draw tothe anode all I of the electrons liberated by the cathode by the lightfalling upon it. Thus the potential drop across the resistor itil willprovide an accurate measure of the illumination on photoelectric celltill even if line voltage variations occur. It should be noted,therefore, that since a constant voltage is maintained across theslidewire resistor 8i and the system is balanced when the potential dropacross the effective portion of the slidewire is equal in value to thepotential drop across resistor I68, the position of the contact .i-twhich the system. is balanced will provide an accurate measure of theillumination on cell IIll. if desired, an induction disc generator Milmay be provided as in the previous arrangements for checking the speedof the motor during rebalancing so thatovershooting and consequenthunting will be minimized. If this feature is desired the induction discprimary winding may be energized from a transformer secondary winding 3aas shown in Fig. 1 and the windings and 5H may be connected in circuitwith the grid! of valve I5I and the resistor I65 in the conductor IM, asshown in Fig. 3 or they may be connected in circuit between the grid 91and the lower end of resistor 88 as shown in Fig. 2. In either case, theinduction disc generator will introduce a countervcltage into thenetwork as the motor gains speed and rotates the induction disc duringrebalancing which will tend to effect energi- Zation of the motor in theopposite direction, and when the flow of unbalance current through theresistors 87 and 88 has been reduced to a small value, thiscountervoltage will be predominant and quickly check. the motor.

In Fig. 5, I have illustrated more or less diagrammatically amodification of the arrangement of Fig. l which is of special utility indecreasing the sensitivity of the instrument at relatively low values oftemperature. In this arrangement it will be noted the point ofconnection of conductor tail to the slidewire resistor 6| is fixed and aresistor I88 employed in lieu of the resistor I68 of Fig. 4, is variedin effecting rebalan-ce of the system upon a change in the temperatureof the incandescent body 2. As the photoelectric current varies it willbe noted the resistance of resistor is varied until the potential droptherea-cross is restored to its initial value. In thus effectingrebalance, the resistance of resistor itt is seen to vary inversely withthe photoelectric current and it will be ap-- parent that when thephotoelectric current is small a larger change in. resistance i58 willbe required to rebalance than when the netoelectric current large. if arecord-er chart is arranged in relation to the resistor 666 it will beseen that low ten'lperatures large changes in resistance if wili berequired to balance small changes in ctoelectric current so that arelatively narrow range of temperatures will be recorded over chart.

[is shown the resist-or i68 of a liquid type consisting of a suitablecylinder in which a suitable liquid, such, for e .mple, as a dilutesolution of sulphuric is contained. and a conductor I'll which i adaptedto he moved into and out of the liquid when is desired to vary theeffective resistance in circuit. The conductor llli may be moved of thesolution in any convenient in balance the system upon variation in ti.nation on the photoelectric cell iii, and as s is mechanically connectedto a it?) with which a pinion W3, driven through suitable gearing by thereversible electrical motor 938, is adapted to cooperate. If desired apen may be connected to the rack H2 and arranged in suitable relation toa recorder chart to thereby provide a record of the variations in thetemperature of the incandescent body 2.

In Fig. 6, I have illustrated a further modification or" the device ofFig, 1 in which the ener-gy supplied to lamp 4 is measured by anarrangement including a resistor H4 which may be connected in serieswith the lamp 4 in the cathode circuit of valve 2?, and is alsoconnected in a potentiometer circuit including a to rbattery N5 ofconstant voltage, a selenium cell H6, and the primary winding I'I'I of atransformer H8. The potentiometer circuit may be traced from the top endof resistor I'M as seen in Fig. 6, conductor I86, battery I15, seleniumcell H5, transiormer primary Winding ITI, conductor I85, and contact I82which is arranged to be moved along the resistor I H. Thus, thepotential drop produced across a portion of the resistor I14 is arrangedto be opposed to and normally balanced by the electromotive force ofbattery I". It will be understood a voltage compensating bridge illsimilar to that shown in Fig. I may be provided and a suitable portionof the potential drop across resistor I may be employed in lieu of thebattery Ill, if desired.

When the opposed voltages become unequal in value, unbalance currentflows in the potentiometer circuit through the selenium cell III and thetransformer primary winding Ill in one direction or the other dependingon which voltage is the greater.

In this modification the unbalance current flow through the seleniumcell I" is converted into a pulsating current flow by subjecting theselenium cell to a light source of varying intensity. It hcharacteristic of a selenium cell to vary in electrical resistance inaccordance with the amount of light transmitted to it so that when thecell is subjected to regular flashes of light, the current flowconducted thereby will be intermittent. When this pulsating currentthrough the transformer primary winding I'll an alternating voltage ofcorresponding Phase and magnitude will be induced in the secondary I"thereof which may be readily amplified by the use of one of variousknown forms of electronic amplifiers.

The variable source of light referred to may advantageously take theform of a neon lamp ill having its terminals connected to thealternating current supply conductors L and L so that the lamp emitslight varying in intensity as the current supplied by the conductors Land 11' varies in direction and magnitude. Desirably one electrode ofthe neon lamp I is shielded from the selenium cell I'Ii so that a flashof light will be received thereby only during one-half of each completecycle of the alternating current supplied by the conductors LI and L'-One electrode of lamp I may be in the form of a relatively small plateand the other electrode in the form of a larger plate disposed betweenthe smaller plate and the selenium cell I" so that the larger platecompletely shields the smaller plate from the cell I" and the latterwill thereby be responsive to the source I during only one-half of eachcycle. The purpose of so shielding one electrode of the lamp I is toPermit selective energization of reversible motor Ill when the lamp IIIis energized from the same source of alternating current supply as themotor phase winding I. As shown, a suitable converging lens I may beinterposed between mamp I" and the selenium cell I'll, if de- Thus, whenunbalance potentiometric currents flow through the selenium cell 1'",the latter will transform the unbalance currents into pulsating currentsof regular frequency which may be readily amplified, as for example, byan electronic valve I and a cooperating pair of electronic valves IIIand III which may be identical with the correspondingly identified partsof Fig. 1. The electronic valve III is a heater yp triode and includesan anode III, a cathode I81,

and heater current may also be supplied to the filament I from asuitable secondary winding on the transformer 68. A condenser I93connected across the transformer secondary winding I19 reduces theimpedance of the latter to the fiow of induced currents therethrough,and renders the corresponding portion of the circuit resonant so thatthe voltage on the control grid Ill of valve I" will be precisely in, or180 out of phase with the voltage of the supply lines Li and L. Acondenser I84 connected across the transformer primary winding I9Ireduces the impedance of the latter and thereby increases the magnitudeof the current iiow in the transformer secondary winding I82.

Thus, when an alternating voltage is induced across the terminals oftransformer secondary winding I19, pulsating current will be conductedby the valve I85 and the transformer primary winding Ill with th resultthat an alternating voltage will be induced across the transformersecondary winding I92. pressed on the input circuits of valves III andII'! where it is further amplified and the amplified quantity is appliedto the motor phase winding III as described in connection with Fig. i.The shaft of motor I30 may be connected through suitable gearing in anyconvenient manner to the contact I82 so that when the motor is energizedfor rotation, the contact I02 will be adjusted along the resistor ill inthe proper direction to reduce the unbalance current flow in thepotentiometer circuit to zero. If desired, an induction disc generatorI40 may be connected in the potentiometer circuit as shown in Fig. 1 orin the input circuits of either valve III or valves III and III forchecking the speed of motor I30 during rebalancing to thereby minimizeovershooting and consequent hunting.

If desired, a chart I31 as shown in Fig. 1 may be arranged incooperative relation with the slidewire resistor I'll and a pen may bemounted on the contact I02 for recording the variations in thetemperature of the incandescent body 2.

It will be apparent that motor I30 may be employed to operate a controlvalve or a rheostat for governing the application of an agent to afurnace for producing heat, for example, to the radiant energy of whichthe photoelectric cell I or I'll is responsive, or another motordesirably operated together with the motor I30 may be so employed. Forexample, as shown in Fig. 7, a furnace 2, to the radiant energy of whichthe photoelectric 'cell I or I10 is responsive, is heated by a resistorINA which is connected to electric supply conductors L and L through arheostat INA, the adjustment of which is effected by a motor I95. Themotor I" may be exactly like the motor I30, and in -Fig. '1 is shown ashaving one phase winding connected to the conductors I32 and I83 whichare connected to the terminals of transformer secondary winding I25, andthe other phase winding is connected through a condenser I96 to thesupply lines L and L The mechanical connection of the rheostat INA tothe motor I is such as to increase and decrease the supply of electriccurrent to the resistor INA as the radiant energy to which thephotoelectric cell I or I" is responsive drops below or rises above apredetermined -level.

Subject matter disclosed in this application and not claimed is beingclaimed in my copending application Serial No. 162,474, filed Thisvoltage is im- September 4, 1937. That subject matter consists ofsubject matter generally like that disclosed in this application, butdifiers therefrom in. that the measuring circuit is not limitedspecifically to a potentiometric circuit as in this application butinstead is more broadly directed to electrical networks.

While in accordance with the provisions of the statutes, 1 haveillustrated; and described the best form of embodiment of my inventionnow known to me, it will be apparent to those skilled in the art thatchanges may be made in the form of the apparatus disclosed withoutdeparting from the spirit of my invention as set forth in the oppeudedclaims and that in some crises certain features of my invention may heto advantage without o corresponding use of other features.

Having now described my invention, as new and desire to secure byPotent, is: V

l. Photometric measuring apparatus including the combination of lightsensitive device responsive to change magnitude of a variable condition,a second light sensitive device, an electric lump arranged to illuminatesaid second mentioned light sensitive device, means under oontrolor saidfirst mentioned light sensitive device for energizing sold lamp so thatthe illumination of said second light sensitive device is definitelyrelated to the; of the first mentioned What 12 L tters escapes lightsensitive dcvlce responsive to change in magnitude of a. variablecondition for producing an unbalanced electric current of magnitudecorresponding with said change in said potentiometric measuring circuit,an impedance in said circuit adapted to be adjusted to reduce saidelectric current, o. reversible electrical motor adapted to adjust saidimpedance, electrical valve means for continuously controlling saidreversible electrical motor to reduce said unbalanced electric currentsubstantially to zero, and operative conlight sensitive device, s.resistor connected in' circuit with. said lamp and said energizingmeans, means for opposing the unidirectional potential drop producedacross said resistor by the lamp energizing moons to a potential droprepresentative of a previous value of the conclltion, means fortranslating the resultant of sold opposed potential drops into analternet lug voltage whose phase and magnitude are determined by thepolarity and magnitude of said resultant unidirectional voltage, saidlast mentioned means including 2. pair of electronic valves each havingan anode, cathode, and a grid, a connection between said cathodes, atransformer, a connection between each of sold anodes and a respectiveend of one winding of said transformer, an alternating voltage supplysource, a connection from one terminal of said alternating voltagesupply wurce to a center tap on said transformer winding and aconnectlon from the other terminal of said alternating pp y source tosaid cathodes, similar resistors connected between each of said gridsand said cathodes, means for passing sold resultant unbalance currentthrough said resistors, and a reversible electrical motor selectivelyenergized by said alternating voltage adapted to vary the magnitude ofsaid second potential drop to reduce said unbalance voltage.

2. Photometric measuring apparatus comprisiing the combination with apotentiometric measuring circuit of a. light sensitive device responsiveto changes in a variable condition for producing a. flow of unbalancedelectric current in said potentlometric measuring circuit, a. deviceadapted to reduce said unbalanced electric current, electl'ichl valvemeans for continuously controlling the actuation of said device inaccordance with the magnitude of said condition,

and operative connections to directly control said electrical valvemeans by said unbalanced electric current.

3 Photometric measuring apparatus comprisin: a potentiometric measuringcircuit and o.

nections to directl}, control said electrical valve means by saidunbalanced electric current.

Photometric measuring apparatus comprising a potehtiometric measuringcircuit and a light sensitive device responsive to change in magnitudeof a, condition for producing an unbalanced electric current in saidcircuit, a reversible electric motor adapted to reduce said imbalancedelectric current, electrical valve means for continuously controllingsaid reversible electrical motor to reduce said unbalanced electriccurrent substantially to zero, operative connection to directly controlsaid electrical valve means by said unbalanced electric current, ameasuring element actuated in accordance with the movements of saidmotor, and means responsive to the speed of said motor adopted tocontrol its deceleration to ensure reduction of said unbalanced currentto zero without overshooting.

5. Photometric measuring apparatus comprising a. potentlometricmeasuring circuit and a light sensitive device responsive to change inmagnitude of at variable condition for producing an unbalanced electriccurrent of polarity and magnitude in said circuit corresponding to thedirection and extent of said change, an impedance in said circuitadapted to be adjusted to re- I duce said unbalanced electric current,a. reversible electrical motor adopted to adjust said impedance, meansincluding electrical valve means having an input circuit and an outputcircuit for continuously controlling said reversible electrical motor inaccordance with the polarity and magnitude of said unbalanced currentand with the rate of reduction of said unbalanced electric current asefiected by adjustment of said impedonce to reduce said unbalancedcurrent substantially to zero, a. connection between said reversibleelectrical valve means, and operative connections to directly controlthe input circuit of said electrical valve means by said unbalancedelectric current.

6. Photometric measuring apparatus including the combination of a lightsensitive device responsive to change in magnitude of a variablecondition, a second light sensitive device, means for illuminating saidsecond light sensitive device so that its illumination is definitelyrelated to that of the first mentioned light sensitive device, meansresponsive to changes in the energy used by the first mentioned means,said last mentioned means including a potentiometrlc network in. whichan unbalanced electric current flow is adapted to be produced uponchange in the energy used by said first mentioned means, a deviceadapted to reduce said unbalanced electric current flow, electricalvalve means for continuously controlling the actuation of said device.and operative connections to directly control said electrical valvemeans by said unbalanced electric current.

7. Photometric measuring apparatus including the combination oi a lightsensitive device responsive'to change in magnitude oi a variablecondition, a second light sensitive device, means ior illuminating saidsecond light sensitive device so that its illumination is substantiallythe same as that of the first mentioned light sensitive device, meansfor producing a potential drop representative of the magnitude oi theillumination on the second mentioned light sensitive device, means foropposing said potential drop to a second potential drop representativeoi a previous value oi the illumination of said second mentioned lightsensitive device to thereby produce an electric current fiow when saidpotential drops are unequal in magnitude, a reversible electrical motoradapted to vary said second mentioned potential drop to equalize saidpotential drops and thereby reduce said electric current flowsubstantially to zero, electrical valve means for continuouslycontrolling said reversible electrical motor, and operative connectionsto directly control said electrical valve means by said electriccurrent.

8. Measuring apparatus including-the combination oi a Wheatstone bridgethe resistance oi one arm oi which is responsive to change in magnitudeoi a variable condition thereby unbalancing said bridge, a second arm,means ior varying the resistance oi said second arm so that itsresistance is definitely related to that oi the first mentioned armincluding energy controlling means responsive to bridge unbalance, saidlast mentioned means including a potentiometricnetwork in which anunbalanced electric current fiow having a polarity and magnitudedetermined by the sense and extent of change in resistance oi saidsecond mentioned arm is adapted to be produced, a device adapted toadjust said potentiometric circuit to reduce said unbalanced electriccurrent fiow, electrical valve means ior continuously controlling theactuation of said device, and operative connections to directly controlsaid electrical valve means by said unbalanced electric current.

9. Measuring apparatus including the combination oi a Wheatstone bridgeadapted to be unbalanced in accordance with the change in magnitude oi avariable condition, means for measuring said unbalance including meansfor producing a potential drop across an impedance variable inaccordance with bridge unbalance and a seli balancing potentiometricnetwork adapted to measure said potential drop and to be unbalanced inaccordance with the sense and extent oi the change thereof and therebyadapted to have an unbalanced electric current flow produced therein oia polarity and magnitude in accordance with the sense and extent oi thechange in said potential drop, a device adapted to eiiect rebalance. oi.said 'potentiometric network and thereby reduce said unbalanced electriccurrent substantially to zero, electrical valve means ior continuouslycontrolling the actuation oi said last mentioned device, and operativeconnections tent oi the change of energization oi the second mentioneddevice and thereby adapted to have an unbalanced electric current fiowproduced therein oi a polarity and magnitude inaccordance with the senseand extent of the change oi energization of the second mentioned device,a device adapted to efi'ec't rebalance oi said potentiometric networkand thereby reduce said unbalanced electric current, electrical valvemeans for continuously controlling said last mentioned device, andoperative connections to directly.control said electrical valve meansbysaid unbalanced electric current.

11. Measuring apparatus including the combination of a device adapted toproduce an cil'ect responsive to change of magnitude of a variablecondition, means ior producing an efi'ect variable in accordance withthe first mentioned effect and definitely related thereto, meansactuated in response to changes in said second efiect, said lastmentioned means including a potentiometric network adapted to beunbalanced in accordance with the sense and extent of the change in saidsecond eil'ect and thereby adapted to have an unbalanced electriccurrent fiow produced therein of a polarity and magnitude in accordancewith the sense and extent oi the change oi actuation oi the secondmentioned means, a device adapted to effect rebalance of saidpotentiometric network and thereby reduce said unbalanced electriccurrent, electrical valve means ior continuously controlling said lastmentioned device, and operative connections to directly control saidelectrical valve means by said unbalanced electric current.

12. Photometric measuring apparatus including the combination of a lightsensitive device responsive to change in magnitude of a variablecondition, a second light sensitive device, an electric lamp arranged-to illuminate said second light sensitive device, means under controloi said first mentioned light sensitive device for energizing said lampso that the illumination oi said sec- 'ond light sensitive device isdefinitely related to that of the first mentioned light sensitivedevice. a resistor connected in circuit with said lamp and saidenergizing means, means ior opposing the unidirectional potential dropproduced across said resistor by the lamp energizing means to aunidirectional potential drop representative of a previous value of thecondition to thereby produce an electric current fiow of polarity andmagnitude determined by the resultant oi said potential drops, areversible electrical motor adapted to vary said second mentionedpotential drop to reduce the resultant potential diiierence oi saidpotential drops and thereby to reduce said electric current flow,electrical valve means ior continuously controlling said reversibleelectrical motor, and operative connections to directly control saidelectrical valve means by said electrical current fiow.

13. Photometric measuring apparatus including the combination oi a lightsensitive device cuit with said lamp and said energizing means.

means ior opposing the unidirectional potential drop produced acrosssaid resistor by the lamp energizing means to a potential droprepresentative of a previous value of the condition to thereby producean electric current-flow in one direction or the other when saidpotential drops are unequal. means for translating the resultant of saidopposed potential drops into an alternating voltage whose phase andmagnitude are determined by the polarity and magnitude of said resultantunidirectional voltage, said last mentioned means including a pair ofelectronic valves each having an anode, cathode, and a grid, means toapply an alternating voltage to the anodes of said valves, and operativeconnections to directly control the relative conductivities of saidvalves by said electric current fiow, and a reversible electrical motorselectively energized by said alternating voltage to vary theIna-grittude of said second potential drop to equalize said potentialdrops.

14. Measuring apparatus including the combination of a Wheatstone bridgecircuit adapted to ,e unbalanced in accordance with change in magnitudeof a variable condition, means responsive to bridge circuit unbalance torebalance said bridge circuit and to produce a potential. drop across animpedance variable in accordance with the magnitude of said condition, aself balancing potentioinetric network adapted to measure said aaiaosspotential drop and adapted to have unbalanced electric current flowsproduced therein in accordance with the sense and the extent of thechange of said potential drop, a device adapted to effect rebaiance ofsaid potentiometric network and thereby reduce said unbalanced electriccurrent fiows substantially to zero, electric valve means forcontinuously controlling the actuation of said device, and operativeconnections to directly controi said electrical valve means by saidunbalanced electric current flows.

15. Measuring auparatus comprising a potentiometric measuring circuitand a current producing device responsive to change in magnitude of avariable condition for unloaiancing said potentiornetric measufngcircuit to an extent corresponding with change, an impedance in saidcircuit adapted to be adjusted to rebalance said circuit, a reveleiectrical motor adapted to in trance, electrical valve means reslrvsive to sci. potentiornetric unbalance for co tiuuouslv control ng saidl'notor, said electricsv alve having an output circuit to reversibleelectrical motor and an inpnt ci cuit, and a connection 05 substantiallyconstant impedance connecting said adjustable innaedance, said deviceand the input circuit of electricai valve means

